Magnetic material and manufacturing method thereof

ABSTRACT

The invention relates to a magnetic material and a method of manufacturing the same. An objective of the invention resides in providing a magnetic material whose dimensional change rate after sintering is low and whose magnetic property is not deteriorated. In order to achieve the objective, a magnetic material according to the invention comprises a great number of ferrite grains (1) which are formed by sintering ferrite raw powder, and shrinkage preventing grains (2) which are interposed among these plural ferrite grains (1) and which are sintered with the ferrite grains (1) in contact with the outer peripheries thereof and are also oxidized with oxygen so as to change into ferrite when they are sintered.

TECHNICAL FIELD

The present invention relates to a magnetic material used for various kinds of electronic parts, and a method of manufacturing the same.

BACKGROUND ART

A conventional magnetic material has been manufactured in the following manner.

First, ferrite raw powder is previously sintered, thereafter, the sintered mass is crushed. Particles obtained by the crushing operation are thereafter compressed in metallic dies. The compact removed from the metallic dies is sintered. Thus, the magnetic material is obtained. However, it is a well-known fact that the magnetic material manufactured in the above-described manner inevitably shrinks during the sintering process, and consequently, the magnetic material is designed considering this shrinkage so that it will have a slightly larger size than a desired size when it is sintered. Therefore, after the sintering process, the magnetic material must be cut to have the desired size and shape. However, since the magnetic material after sintered is extremely hard, a cutting blade wears considerably, and it must be replaced with a new one frequently. As a result, the manufacturing cost increase.

Consequently, there was suggested a magnetic material whose shrinkage during sintering was substantially prevented so that the above-mentioned cutting operation would not be required (cf. JP-A-1-264959). This magnetic material is manufactured by mixing magnetic powder with filling powder which is one kind selected from silicon, titanium and aluminum, compressing this mixture by metallic dies, and thereafter sintering the compact, which has been removed from the metallic dies, in an oxygen atmosphere or in a nitrogen atmosphere. In the case of this conventional magnetic material, oxidization expansion or nitrogenization expansion of the above-mentioned filling powder can decrease shrinkage of the magnetic material during sintering to an extremely small degree. However, silicon, titanium, aluminum or the like which is a non-magnetic material is mixed in the magnetic powder, and consequently, there arises a problem that the magnetic property as the magnetic material is deteriorated.

Thus, an objective of the invention resides in providing a magnetic material whose dimensional change due to sintering is small and whose magnetic property is not deteriorated.

DISCLOSURE OF THE INVENTION

In order to achieve the objective, a magnetic material of the invention comprises a great number of ferrite grains which are formed by sintering ferrite raw powder, and shrinkage preventing grains which are interposed among these plural ferrite grains and which are sintered with the ferrite grains in contact with the outer peripheries thereof and are also oxidized with oxygen so as to change into ferrite when the grains are sintered.

With the above-described structure, a rate of dimensional change of the magnetic material due to sintering is low, and deterioration in the magnetic property does not occur. The reason is that the shrinkage preventing grains are interposed among the ferrite grains and sintered with the ferrite grains in contact with their outer peripheries, thereby preventing shrinkage which is caused by ferrite grains approaching to one another to be bound, as in the case of the conventional magnetic material. Further, the shrinkage preventing grains are oxidized with oxygen and change into ferrite when they are sintered with the ferrite grains, so that a non-magnetic material observed in the conventional magnetic material will not be mixed, and that the magnetic property will not be deteriorated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an enlarged view showing an internal structure of a magnetic material according to one embodiment of the present invention;

FIG. 2 is an enlarged view showing one condition of the magnetic material in a manufacturing process; and

FIG. 3 is a characteristic graph showing a dimensional change rate.

BEST MODE FOR CARRYING OUT THE INVENTION Example 1

A first embodiment of the present invention will be hereinafter described with reference to the attached drawings. FIG. 1 is an enlarged view showing an internal structure of a magnetic material in the embodiment of the invention. The magnetic material has a cylindrical shape or a plate-like shape and comprises a great number of ferrite grains 1 and contraction preventing grains 2 which are located among the great number of ferrite grains 1. The ferrite grains 1 are manufactured by mixing and sintering ferrite raw powder consisting of Fe₂ O₃, NiO, ZnO and CuO, these having a mixing mole ratio of 47.2:15.5:32.1:5.2, crushing the sintered mass, and sintering crushed ferrite particles 1a. Further, the ferrite grains 1 thus obtained are mixed with metal iron powder 2a having a diameter of 5 μm or less, as shown in FIG. 2, which amounts to 20 parts by weight of the ferrite particles 1a, and the mixture is sintered. Thus, the shrinkage preventing grains 2 are formed among the ferrite grains 1, as shown in FIG. 1.

A manufacturing method of the magnetic material comprises a first step of sintering, at 1320° C. for six hours, ferrite raw powder consisting of Fe₂ O₃, NiO, ZnO and CuO which have a mixing mole ratio of 47.2:15.5:32.1:5.2, a second step of forming ferrite particles 1a by crushing the sintered mass into particles having a diameter of 40 μm or less, a third step of forming shrinkage preventing grains 2 by mixing metal iron powder 2a having a diameter of 5 μm or less with the ferrite particles 1a, as shown in FIG. 2, which metal iron powder amounts to 20 pats by weight of the ferrite particles 1a, a fourth step of forming pellets by adding 7 wt % epoxy resin to a mixture of the ferrite particles 1a and the metal iron powder 2a, a fifth step of compressing the formed pellets into an article of a cylindrical shape having an inner diameter of 7 mm, an outer diameter of 12 mm and a thickness of 3 mm under a pressure of 3 ton/cm², and a sixth step of sintering the compact of the cylinder shape in an electric furnace at 1200° C.

In this example, it is a notable characteristic that, as shown in FIG. 2, the metal iron powder 2a for forming the shrinkage preventing grains 2 is located among the great number of ferrite grains 1 in the third step, and this mixture is sintered.

That is to say, by sintering in the sixth step, the metal iron powder 2a thus located is sintered with the ferrite grains 1 in contact with their outer peripheries, and also oxidized with oxygen, so that the metal iron powder 2a formed into the shrinkage preventing grains 2 by sintering will prevent the ferrite grains 1 from approaching to one another to be sintered, as shown in FIG. 1. As a result, the dimensional change rate is extremely low.

In the sixth step, although the ferrite grains 1 are partially sintered with one another, the metal iron powder 2a is located in large gaps among the ferrite grains 1 without fail, as shown in FIG. 2, so that the ferrite grains 1 will be generally sintered with the shrinkage preventing grains 2, to thereby prevent shrinkage.

Moreover, at the time of sintering in the sixth step, the metal iron powder 2a combines with oxygen in the external atmosphere and is oxidized to increase in diameter. When the metal iron powder 2a among the ferrite grains 1 is oxidized and expansion, shrinkage among the ferrite grains 1 is likewise prevented.

When the metal iron powder 2a is sintered with the ferrite grains 1 on the outer peripheries thereof, elements are partially diffused and supplied from the ferrite grains 1, and the above-described oxidization simultaneously takes place, so that the metal iron powder 2a itself changes into ferrite. By the way, reference numeral 3 in FIG. 1 denotes a gap formed after sintering.

FIG. 3 illustrates the relationship between a mixing amount of iron metal powder 2a and a dimensional change rate and a mechanical strength of a magnetic material. As samples, magnetic materials obtained by mixing 0 to 50 parts of metal powder 2a by weight of ferrite particles 1a of metal iron powder 2a with ferrite particles 1a are employed. As shown in FIG. 3, when the mixing amount of the metal iron powder 2a is five parts by weight of ferrite particles or less, the mechanical strength is sufficient, but the effect as the shrinkage preventing grains 2 is so small that the dimensional change rate during sintering is increased. When the mixing amount of metal powder is 40 parts by weight of ferrite particles or more, a sufficient mechanical strength can not be obtained although the dimensional change rate is low. Therefore, the mixing amount of the metal iron powder 2a should preferably be more than five parts by weight of ferrite particles and less than 40 parts by weight of ferrite particles.

Properties of magnetic materials obtained by adding 2 to 30 parts of metals or metallic compounds other than the metal iron powder 2a by weight of ferrite particles, as the shrinkage preventing grains 2, and properties of magnetic materials obtained by adding the metal iron powder 2a are shown in Table 1 for comparison.

                                      TABLE 1                                      __________________________________________________________________________                              ADDITIVE  DIMENSIONAL                                                                             INTIAL    MECHANICAL               SPECIMEN          DIAMETER                                                                              AMOUNT    CHANGE RATE                                                                             MAGNETIC  STRENGTH                 NO.    FILLING POWDER                                                                            (μm)                                                                               (WEIGHT PART)                                                                            (%)      PERMEABILITY                                                                             (Kgf)                    __________________________________________________________________________     1      METAL IRON <5     5         -0.9     270       8.1                      2      METAL IRON <5     10        -0.4     240       7.5                      3      METAL IRON <5     20        -0.0     245       7.3                      4      METAL IRON <5     30        0.2      245       7.3                      5      METAL NICKEL                                                                              <3     5         -0.9     130       8.8                      6      METAL NICKEL                                                                              <3     10        -0.7     120       8.2                      7      METAL NICKEL                                                                              <3     20        0.0      80        7.8                      8      METAL NICKEL                                                                              <3     30        0.2      60        7.5                      9      METAL ZINC <7     7         -0.9     180       8.9                      10     METAL ZINC <7     10        -0.5     110       9.0                      11     METAL ZINC <7     20        0.0      90        9.1                      12     METAL ZINC <7     30        0.6      80        9.0                      13     FeO        <10    5         -0.8     220       6.1                      14     FeO        <10    10        -0.4     150       5.8                      15     FeO        <10    20        0.0      130       6.0                      16     FeO        <10    30        0.0      125       6.3                      __________________________________________________________________________

Specimen No. 1 to 16 are the magnetic materials according to this embodiment obtained by adding various kinds of filling powder (metal iron powder (Fe), metal nickel powder (Ni), metal zinc powder (Zn), iron oxide powder (FeO)).

As clearly understood from Table 1, the optimum mixing amounts for decreasing the dimensional change rates varied in accordance with the kinds of filling powder. However, as the mixing amount of the filling powder increased, the dimensional change rate decreased, and magnetic materials which had the same dimensions as the molded articles could be obtained. Further, in all of specimens No. 1 to 16, sufficient magnetic properties and mechanical strengths could be obtained.

Properties of magnetic materials obtained by mixing various kinds of metals at desired percentages (wt %) as filling powder, and adding 15 to 25 parts of the mixed powder by weight of ferrite particles to ferrite particles are shown in Table 2 for comparison.

                                      TABLE 2                                      __________________________________________________________________________             METAL POWDER MIXING      DIMENSIONAL                                                                              INITIAL   MECHANICAL                SPECIMEN                                                                               PERCENTAGE (wt %)                                                                             MIXING RATE                                                                              CHANGE RATE                                                                              MAGNETIC  STRENGTH                  NO.     IRON NICKEL                                                                              ZINC (WEIGHT PART)                                                                            (%)       PERMEABILITY                                                                             (kgf)                     __________________________________________________________________________     17      40   60   0    25        0.0       190       9.6                       18      40   40   20   20        0.2       225       8.3                       19      40   20   40   20        0.3       210       8.6                       20      40   0    60   20        0.2       200       11.5                      21      20   80   0    25        0.0       105       10.2                      22      20   60   20   20        0.5       125       7.8                       23      20   40   40   20        0.5       125       7.2                       24      20   20   60   20        0.4       140       7.1                       25      20   0    80   20        0.2       150       10.6                      26      0    80   20   20        0.7       90        6.3                       27      0    60   40   15        0.8       70        6.4                       28      0    40   60   15        0.8       75        6.4                       29      0    20   80   20        0.7       85        7.4                       __________________________________________________________________________

Specimens No. 17 to 29 are the magnetic materials according to this embodiment obtained by adding various kinds of filling powder (mixed powder of metal iron powder, metal nickel powder and metal zinc powder).

As clearly understood from Table 2, although the filling powder is mixed powder, magnetic materials whose dimensional change rates are low and which have sufficient magnetic properties and mechanical strengths could be obtained.

Example 2

A second embodiment of the present invention will be now described. Structures of magnetic materials and their manufacturing methods which are used in Example 2 are substantially the same as those of Example 1 so that explanations thereof will be omitted. Properties of the magnetic materials in which filling powder for forming contraction preventing grains 2 is varied will be described below.

Properties of magnetic materials obtained by adding 20 to 25 parts of filling powder by weight of ferrite particles to ferrite particles for forming ferrite grains 1 are shown in Table 3 for comparison.

                                      TABLE 3                                      __________________________________________________________________________             METAL POWDER MIXING      DIMENSIONAL                                                                              INITIAL   MECHANICAL                SPECIMEN                                                                               PERCENTAGE (wt %)                                                                             MIXING RATE                                                                              CHANGE RATE                                                                              MAGNETIC  STRENGTH                  NO.     IRON NICKEL                                                                              ZINC (WEIGHT PART)                                                                            (%)       PERMEABILITY                                                                             (kgf)                     __________________________________________________________________________     30      80   20   0    25        0.0       340       10.8                      31      80   0    20   25        0.0       380       11.4                      32      60   40   0    25        0.0       300       10.5                      33      60   20   20   25        0.1       360       11.0                      34      60   0    40   20        0.1       310       10.2                      35      63   13   24   20        0.1       460       10.3                      __________________________________________________________________________

Specimens No. 30 to 35 are the magnetic materials according to this embodiment obtained by adding various kinds of filling powder (mixed powder of metal iron powder (Fe), metal nickel powder (Ni) and metal zinc powder (Zn)) at different mixing percentages.

As clearly understood from Table 3, magnetic materials which had the same dimensions as the compacts could be obtained, and initial magnetic permeabilities were larger than those of the specimens of Example 1.

Properties of magnetic materials obtained by mixing various kinds of metals or various kinds of metallic oxides as filling powder at percentages substantially equal to the structural percentage of powder metallic elements which constitute the sample 35, and adding 25 weight parts of the mixed powder to ferrite particles for forming ferrite grains 1 are shown in Table 4 for comparison.

                                      TABLE 4                                      __________________________________________________________________________                         DIMENSIONAL                                                                              INITIAL    MECHANICAL                            SPECIMEN                                                                               METAL OR    CHANGE RATE                                                                              MAGNETIC   STRENGTH                              NO.     METALLIC OXIDE                                                                             (%)       PERMEABILITY                                                                              (kgf)                                 __________________________________________________________________________     36      METAL IRON, 0.0       480        10.8                                          NICKEL OXIDE,                                                                  ZINC OXIDE                                                             37      IRON OXIDE, 0.0       470        11.3                                          METAL NICKEL,                                                                  ZINC OXIDE                                                             38      IRON OXIDE, 0.0       470        10.4                                          NICKEL OXIDE,                                                                  METAL ZINC                                                             __________________________________________________________________________

Specimens No. 36 to 38 are the magnetic materials according to this embodiment obtained by adding various kinds of filling powder (mixed powder of metal iron powder (diameter: 5 μm), metal nickel powder (diameter: 3 μm), metal zinc powder (diameter: 7 μm), nickel oxide powder (diameter: 1 μm), zinc oxide powder (diameter: 1 μm) and iron oxide powder (diameter: 1 μm)) at different mixing percentages. Among these specimens, the specimen 36 contains metal iron powder, nickel oxide powder and zinc oxide powder at a weight ratio of 57.6:15.0:27.4. The specimen 37 contains metal nickel powder, iron oxide powder and zinc oxide powder at a weight ratio of 9.7:67.8:22.5. The specimen 38 contains metal zinc powder, nickel oxide powder and iron oxide powder at a weight ratio of 18.5:69.0:12.5.

In the specimens 36 to 38, as clearly understood from Table 4, magnetic materials which had the same dimensions as the compact could be obtained, and they had sufficient magnetic properties and mechanical strengths.

It should be noted that in the specimens 30 to 38, only the X-ray diffraction peaks which indicated spinel crystal structures were observed, and X-ray diffraction peaks which indicated crystal structures other than the spinel crystal structures observed in the samples of Example 1 were not observed.

This results implies that a sintered product only comprises a ferrite sintered material having a spinel crystal structure so that a sintered material whose initial magnetic permeability is more excellent can be obtained.

Example 3

A third embodiment of the present invention will be now described. Structures of a magnetic material and its manufacturing method which are used in Example 3 are substantially the same as those of Example 1 so that explanations thereof will be omitted. Properties of the magnetic material in which filling powder is varied will be described below.

Properties of a magnetic material obtained by mixing metal iron powder (diameter: 5 μm), nickel oxide powder (diameter: 1 μm), zinc oxide powder [diameter: 1 μm) and copper oxide powder (diameter: 1 μm)) as filling powder at a weight ratio of 55.8:12.2:27.6:4.4 so as to have substantially the same composition as ferrite grains 1, and adding 25 parts of mixed powder by weight of ferrite particles of the mixed powder to ferrite particles for forming the ferrite grains 1 are shown in Table 5 for comparison.

                  TABLE 5                                                          ______________________________________                                                             INITIAL                                                           DIMENSIONAL  MAGNETIC   MECHANICAL                                      Specimen                                                                              CHANGE RATE  PERME-     STRENGTH                                        No.    (%)          ABILITY    (Kgf)                                           ______________________________________                                         39     0.0          650        12.8                                            ______________________________________                                    

As clearly understood from Table 5, the specimen had a uniform structure, and consequently, a sintered magnetic material having a larger initial magnetic permeability than that of Example 2 was obtained. Further, even if the oxides which constitute the above-mentioned filling powder are metal powder, a similar effect can be obtained.

Example 4

A fourth embodiment of the present invention will be now described. Magnetic materials used in Example 4 are obtained by mixing 50 parts of filling powder containing at least Zn-system ferrite by weight of ferrite particles with ferrite particles for forming ferrite grains 1 which are formed of ferrite raw powder having a mixing mole ratio of 47.8:15.2:32.0:5.0. Structures of the magnetic materials and their manufacturing methods are substantially the same as those of Example 1 so that explanations thereof will be omitted. Properties of the magnetic materials will be described below.

Properties of the magnetic materials according to this embodiment are shown in Table 6 for comparison.

                                      TABLE 6                                      __________________________________________________________________________                                         DIMEN-                                            FILTING POWDER (wt %)        SIONAL                                                                               DEFORMA-                                                                              INITIAL                                                                               MECHA-                                      METAL METAL    CHANGE                                                                               TION   MAGNETIC                                                                              NICAL                  SPECIMEN                                                                              Zn-SYSTEM FERRITE                                                                            POWDER                                                                               COMPOUND RATE  AMOUNT PERME- STRENGTH               NO.    ZnFe.sub.2 O.sub.4                                                                  Ni0.1Zn0.9Fe.sub.2 O.sub.4                                                              Fe Ni NiO                                                                               ZnO                                                                               CuO                                                                               (%)   (μm)                                                                               ABILITY                                                                               (kgf)                  __________________________________________________________________________     40     70.0 0.0      16.2                                                                              0.0                                                                               10.3                                                                              0.0                                                                               3.5                                                                               0.0   0      660    13.1                   41     70.0 0.0      16.2                                                                              10.3                                                                              0.0                                                                               0.0                                                                               3.5                                                                               -0.1  2      660    12.8                   42     64.9 0.0      15.0                                                                              0.0                                                                               20.1                                                                              0.0                                                                               0.0                                                                               0.0   1      610    12.3                   43     0.0  76.5     12.2                                                                              0.0                                                                               7.8                                                                               0.0                                                                               3.5                                                                               -0.1  3      650    12.7                   44     0.0  76.5     12.2                                                                              7.8                                                                               0.0                                                                               0.0                                                                               3.5                                                                               0.0   1      640    12.3                   45     0.0  80.6     10.2                                                                              0.0                                                                               9.2                                                                               0.0                                                                               0.0                                                                               0.0   1      620    12.1                   COMPARA-                                                                              --   --       -- -- -- -- -- -3.0  50     630    12.5                   TIVE                                                                           SPECIMEN                                                                         1                                                                            COMPARA-                                                                              0.0  0.0      56.4                                                                              0.0                                                                               12.0                                                                              27.5                                                                              4.1                                                                               0.0   20     650    12.8                   TIVE                                                                           SPECIMEN                                                                        2                                                                             __________________________________________________________________________

Specimens No. 40 to 45 are the magnetic materials according to this embodiment, a comparative specimen 1 is a magnetic material with which filling powder is not mixed in this embodiment, and a comparative specimen 2 is a magnetic material obtained by mixing metal iron powder and metal oxides to have substantially the same composition as ferrite particles without mixing Zn-system ferrite, and mixing 25 weight parts of this filling powder with the ferrite particles for forming ferrite grains 1 in this embodiment.

As clearly understood from Table 6, magnetic properties and mechanical strengths were substantially the same in the specimens 40 to 45, the comparative specimen 1 and the comparative specimen 2. In the comparative specimen 1 to which filling powder was not added, dimensional contraction of 3% occurred during sintering. On the other hand, the dimensional shrinkage rates could be extremely decreased in the samples 40 to 45 and the comparative specimen 2 to which filling powder was added. Further, the samples 40 to 45 and the comparative specimen 2 were compared. In the case of the specimens 40 to 45, because filling powder included Zn-system ferrite, the optimum mixing amount of the filling powder was increased, but a deformation amount was improved and lower than that of the comparative specimen 2. It was found that mixing of Zn-system ferrite powder is effective for manufacturing a magnetic material of a high dimensional accuracy.

Initial magnetic permeabilities, dimensional change rates, tensile strengths and deformation amounts in Example 1, Example 2, Example 3 and Example 4 were calculated and measured as follows.

In order to measure the initial magnetic permeability, each sample was prepared by winding an insulation tape around the above-described annular magnetic material once, and winding an insulation copper wire having a diameter of 0.26 mm over the entire circumference of the magnetic material uniformly once. Next, a self-inductance L at 1 MHz was measured by an impedance analyzer when a measuring magnetic field strength was 0.8 (A/m) or less, and an initial magnetic permeability was calculated.

The dimensional change rate was obtained by measuring a diameter of the annular molded article before heat treatment and a diameter of the annular magnetic material after heat treatment, and calculating a rate of the diameters. A minus denotes contraction.

In order to measure the tensile strength, two thin wires were inserted in the annular magnetic material once, and after one of the wires was fixed, the other wire was drawn in the vertical direction at a speed of 5 mm/min or less. A tensile load at the moment of breakage of the magnetic material was measured. The deformation amount was obtained by measuring the maximum value and the minimum value of the diameter of the annular magnetic material after sintering, and calculating a difference between the values.

INDUSTRIAL APPLICABILITY

As described heretofore, according to the present invention, the magnetic material comprises a great number of ferrite grains which are formed by sintering ferrite raw powder, and shrinkage preventing grains which are interposed among these plural ferrite grains and which are sintered with the ferrite grains in contact with the outer peripheries thereof and are also oxidized with oxygen so as to change into ferrite when the grains are sintered.

With the above-described structure, there can be provided a magnetic material whose dimensional change rate due to sintering is low and whose magnetic property is not deteriorated. 

We claim:
 1. A method of manufacturing a Ni-Zn ferrite magnetic material, which comprises:a first step of preparing a ferrite powder by sintering a ferrite raw powder and crushing the sintered powder; a second step of mixing the ferrite powder obtained in the first step and a filling powder with each other; a third step of forming the mixture obtained in the second step to prepare a formed article; and a fourth step of sintering the article obtained in the third step, wherein the filling powder in the second step contains at least one metal of Fe, Ni, and. Zn, and the filling powder in the formed article reacts with oxygen so as to change into shrinkage preventing grains during sintering in the fourth step.
 2. A method of manufacturing a Ni-Zn ferrite magnetic material according to claim 1, whereinthe filling powder in the second step is a mixture of Fe powder and 20 to 40 weight percent Ni powder, and the filling powder in the formed article reacts with oxygen so as to change into shrinkage preventing grains during sintering in the fourth step at such a temperature that the shrinkage preventing grains have an Fe spinel crystal structure.
 3. A method of manufacturing a Ni-Zn ferrite magnetic material according to claim 1, whereinthe filling powder in the second step is a mixture of Fe powder and 20 to 40 weight percent Zn powder, and the filling powder in the formed article reacts with oxygen so as to change into shrinkage preventing grains during sintering in the fourth step at such a temperature that the shrinkage preventing grains have an Fe spinel crystal structure.
 4. A method of manufacturing a Ni-Zn ferrite magnetic material according to claim 1, whereinthe filling powder in the second step is a mixture comprising Fe-containing powder, Ni-containing powder and Zn-containing powder, such that the metal of the filling powder is 60 to 80 weight percent Fe, 13 to 20 weight percent Ni, and 20 to 40 weight percent Zn, and the filling powder in the formed article reacts with oxygen so as to change into shrinkage preventing grains during sintering in the fourth step at such a temperature that the shrinkage preventing grains have an Fe spinel crystal structure.
 5. A method of manufacturing a Ni-Zn ferrite magnetic material according to claim 4, whereinthe filling powder in the second step is a mixture of Fe powder such that the metal of the filling powder is 60 to 80 weight percent Fe, NiO powder such that the metal of the filling powder is 13 to 20 weight percent Ni, and ZnO powder such that the metal of the filling powder is 20 to 40 weight percent Zn.
 6. A method of manufacturing a Ni-Zn ferrite magnetic material , according to claim 4, whereinthe filling powder in the second step is a mixture of Fe₂ O₃ powder such that the metal of the filling powder is 60 to 80 weight percent Fe, Ni powder such that the metal of the filling powder is 13 to 20 weight percent Ni, and ZnO powder such that the metal of the filling powder is 20 to 40 weight percent Zn.
 7. A method of manufacturing a Ni-Zn ferrite magnetic material according to claim 4, whereinthe filling powder in the second step is a mixture of Fe₂ O₃ powder such that the metal of the filling powder is 60 to 80 weight percent Fe, NiO powder such that the metal of the filling powder is 13 to 20 weight percent Ni, and Zn powder such that the metal of the filling powder is 20 to 40 weight percent Zn.
 8. A method of manufacturing a Ni-Zn ferrite magnetic material, which comprises:a first step of preparing a ferrite powder by sintering a ferrite raw powder and crushing the sintered powder; a second step of mixing the ferrite powder obtained in the first step and a filling powder with each other; a third step of forming the mixture obtained in the second step to prepare a formed article; and a fourth step of sintering the article obtained in the third step, wherein the filling powder in the second step comprising Fe powder and having generally the same composition as the ferrite raw powder, and the filling powder in the formed article reacts with oxygen so as to change into shrinkage preventing grains during sintering in the fourth step at such a temperature that the shrinkage preventing grains have an Fe spinel crystal structure.
 9. A method of manufacturing a Ni-Zn ferrite magnetic material according to claim 8, whereinthe filling powder in the second step is a mixture comprising at least one of Fe powder and Ni powder as a metallic powder, at least one of NiO powder and CuO powder as a metal oxide, Zn ferrite powder which contains up to 0.14 atom percent of Ni, mixed with one another to have generally the same composition as the ferrite raw powder, and the filling powder in the formed article reacts with oxygen so as to change into shrinkage preventing grains during sintering in the fourth step at such a temperature that the shrinkage preventing grains have an Fe spinel crystal structure.
 10. A method of manufacturing a Ni-Zn ferrite magnetic material according to claim 1, wherein the powder in the filling powder is at least one of Fe, FeO, Ni, and Zn powders.
 11. A method of manufacturing a Ni-Zn ferrite magnetic material according to claim 4, wherein the filling powder is a mixture of 60 to 80 weight percent of Fe powder, 13 to 20 weight percent of Ni powder, and 20 to 40 weight percent of Zn powder.
 12. A method of manufacturing a Ni-Zn ferrite material according to claim 1, wherein the magnetic material contains about 5 to about 50 parts by weight of the shrinkage preventing grains.
 13. A method of manufacturing a Ni-Zn ferrite magnetic material, which comprises:a first step of preparing a ferrite powder by sintering a ferrite raw powder and crushing the sintered powder; a second step of mixing the ferrite powder obtained in the first step and a filling powder with each other; a third step of forming the mixture obtained in the second step to prepare a formed article; and a fourth step of sintering the article obtained in the third step, wherein the filling powder in the second step contains at least one metal of Fe, Ni and Zn, and the filling powder in the formed article reacts with oxygen so as to change into shrinkage preventing grains during sintering in the fourth step, the sintering being carried out at a lower temperature than that of calcining in the first step.
 14. A method of manufacturing a Ni-Zn ferrite magnetic material, which comprises:a first step of preparing a ferrite powder by sintering a ferrite raw powder and crushing the sintered powder; a second step of mixing the ferrite powder obtained in the first step and a filling powder with each other; a third step of forming the mixture obtained in the second step to prepare a formed article; and a fourth step of sintering the article obtained in the third step, wherein the filling powder in the second step contains at least one metal of Fe, Ni and Zn each particle of which has a diameter smaller than 10 μm, and the filling powder in the formed article reacts with oxygen so as to change into shrinkage preventing grains during sintering in the fourth step.
 15. A method of manufacturing a Ni-Zn ferrite magnetic material according to claim 13, whereinthe filling powder in the second step is a mixture of Fe powder and 20 to 40 weight percent Ni powder, and the filling powder in the formed article reacts with oxygen so as to change into shrinkage preventing grains during sintering in the fourth step at such a temperature that the shrinkage preventing grains have an Fe spinel crystal structure.
 16. A method of manufacturing a Ni-Zn ferrite magnetic material according to claim 12, whereinthe filling powder in the second step is a mixture of Fe powder and 20 to 40 weight percent Zn powder, and the filling powder in the formed article reacts with oxygen so as to change into shrinkage preventing grains during sintering in the fourth step at such a temperature that the shrinkage preventing grains have an Fe spinel crystal structure.
 17. A method of manufacturing a Ni-Zn ferrite magnetic material according to claim 13, whereinthe filling powder in the second step is a mixture comprising Fe-containing powder, Ni-containing powder and Zn-containing powder, such that the metal of the filling powder is 60 to 80 weight percent Fe, 13 to 20 weight percent Ni, and 20 to 40 weight percent Zn, and the filling powder in the formed article reacts with oxygen so as to change into shrinkage preventing grains during sintering in the fourth step at such a temperature that the shrinkage preventing grains have an Fe spinel crystal structure.
 18. A method of manufacturing a Ni-Zn ferrite magnetic material according to claim 13, whereinthe filling powder in the second step is a mixture of Fe powder such that the metal of the filling powder is 60 to 80 weight percent Fe, NiO powder such that the metal of the filling powder is 13 to 20 weight percent Ni, and ZnO powder such that the metal of the filling powder is 20 to 40 weight percent Zn.
 19. A method of manufacturing a Ni-Zn ferrite magnetic material according to claim 13, whereinthe filling powder in the second step is a mixture of Fe₂ O₃ powder such that the metal of the filling powder is 60 to 80 weight percent Fe, Ni powder such that the metal of the filling powder is 13 to 20 weight percent Ni, and ZnO powder such that the metal of the filling powder is 20 to 40 weight percent Zn.
 20. A method of manufacturing a Ni-Zn ferrite magnetic material according to claim 13, whereinthe filling powder in the second step is a mixture of Fe₂ O₃ powder such that the metal of the filling powder is 60 to 80 weight percent Fe, NiO powder such that the metal of the filling powder is 13 to 20 weight percent Ni, and Zn powder such that the metal of the filling powder is 20 to 40 weight percent Zn.
 21. A method of manufacturing a Ni-Zn ferrite magnetic material according to claim 13, wherein the powder in the filling powder is at least one of Fe, FeO, Ni, and Zn powders.
 22. A method of manufacturing a Ni-Zn ferrite material according to claim 13, wherein the magnetic material contains about 5 to about 50 parts by weight of the shrinkage preventing grains.
 23. A method of manufacturing a Ni-Zn ferrite magnetic material according to claim 14, whereinthe filling powder in the second step is a mixture of Fe powder and 20 to 40 weight percent Ni powder, and the filling powder in the formed article reacts with oxygen so as to change into shrinkage preventing grains during sintering in the fourth step at such a temperature that the shrinkage preventing grains have an Fe spinel crystal structure.
 24. A method of manufacturing a Ni-Zn ferrite magnetic material according to claim 14, wherein the powder in the filling powder is at least one of Fe, FeO, Ni, and Zn powders.
 25. A method of manufacturing a Ni-Zn ferrite material according to claim 14, wherein the magnetic material contains about 5 to about 50 parts by weight of the shrinkage preventing grains. 